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Paul Gilman
PV Modules and Inverters not in Database

If you are using SAM's detailed photovoltaic model for a photovoltaic system that uses a module or inverter that is not included in SAM's libraries, you can use the CEC Performance Model with User Entered Specifications or Inverter Datasheet options on the Module and Inverter input pages, respectively. Those options allow you to enter data from a manufacturer's data sheet. Behind the scenes, SAM generates the coefficients it needs to model the equipment.

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SDK's irrad_mode parameter not working


My colleague and I have been using the SDK for the past two years to make energy output predictions for the solar company that we work at. In the past, (more than a year ago) we changed the irrad_mode parameter between 0, 1, and 2 with no problems. Now however, after downloading the most recent release, we find that our energy simulations only work when irrad_mode is set to 0, that is, only when DHI and DNI are input directly. Whenever we try to input GHI directly, our simulation fails.

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Macro "Subarray Layout Optimization" Question


I'm simulating a system on SAM for a desired array size of 6362.35 kWdc. If I check the button "Estimate subarray 1 configuration", the resulting system (33 panels in series x 838 in parallel) has a size that I know does not fit in my area, because of the number of strings in parallel.

I found out that the macro "Subarray Layout Optimization" could optimize the layout given my area constraint. However, when I try running it the following error occurs:

Macro did not finish.
[810] referencing unassigned variable:modules_per_string_npv

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Backtracking with minute data

Dear SAM team,

when setting up backtracking with a minute data file I am seeing strange behaviour of the energy output and the tracking angle. For some month it start at fixed maxiumum angle in the morning until it jumps to cero and than turns back slowly to max angle, remains there before noon and going back to cero during midday.

Please find the screenshots attached.

I would appreciate if you can explain the behaviour and maybe point out a solution to avoid the jump in production.

Thanks a lot

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Weather file SAM 2018 - Europe


I am trying to simulate the solar energy production of a building in Malmö, Sweden with SAM 2018.11.11
I have downloaded the epw weather file from
When I try to simulate I get the following error: the following weather files in the solar resource library appear to have problems.

Could you help me? It is important for me to understand how to download weather files from European cities.

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ERROR - Vmp exceeds the Vdcmax


I got that error message:
Module array voltage Vmp exceeds the Vdcmax (500.00V) of inverter 4326 times.
The maximum Vmp value is 1042.77V at hour 8456.
It is recommended that you reduce the number of modules per string.

The system:
32 x Canadian CS6K-260P
1 x SE-7600A 204 (cec2013)
Modules per string: 32
Strings in parallel: 1

Can you please provide an explanation on the error and how can I overcome it?


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Electricity Bill with system not adding up


When I get modeling results for my electric bill with system, it is higher than the sum of the demand charge with system (flat), energy charge with system, and fixed monthly charge with system. How can this be? I could see it being lower due to net metering credits, but am not sure what could cause it to be higher.

Second, what is the difference between demand charge with system (flat) and demand charge with system (TOU)?

Thanks for your help! I have attached a screenshot of my results if that helps.

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Aswati Sreenivasan
POA data in weather file


I have been trying to create a weather file with 10 min data imported from meteonorm. However when I run the simulation I get the following error "exec fail(pvsamv1): failed to calculate irradiance incident on surface (POA) 1 (code: -101) [y:1900 m:1 d:-2147483648 h:0]
Simulation pvsamv1 failed.". I have although added a column with POA with data from Meteonorm.

Hope for a working solution.


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burakomersaracoglu's picture
Solar Star Projects Validation Research

Dear SAM Users;
I am looking for some coauthors who may want to research on the validation of the Solar Star Projects with the NREL SAM models.
My first scientific journal paper related to the Solar Star Projects PVWatts Model was published in 2018.
"Solar star projects SAM version 2017.9.5 PVWatts version 5 model case study & validation", International Journal of Energy Applications and Technologies, 2018, Vol.5, No.1, 13-28, (

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constraint fail in irradproc


I try to run irradproc-module using irrad_mode = 2 when I receive the following error: "constraint fail: reason length_equal cannot find variable to test against, with 'length_equal=beam' for: diffuse"

Do I need to include beam irradiation data as an input when using irrad_mode=2? I only have global horizontal irradiation and diffuse available.

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